Rotary gas dispersion device for treating a liquid metal bath

ABSTRACT

Rotary injector for injecting gas into a liquid metal includes a drive shaft, stirring means, means for circulating the gas and means for emitting the gas. The emitting means is entirely or partially made of at least one material capable of being wetted by the liquid metal.

FIELD OF THE INVENTION

[0001] The present invention relates to a rotary gas dispersion device for treating a liquid metal bath, in particular aluminium, an aluminium alloy, magnesium or a magnesium alloy. The invention relates more specifically to a rotary injector (“rotor”) designed to inject or disperse at least one treatment gas into a metal in the liquid state.

PRIOR ART

[0002] Liquid aluminium coming from electrolysis cells or remelting furnaces contains dissolved or suspended impurities. The most significant of these impurities are hydrogen, alkaline elements such as sodium or calcium and oxides, especially oxides resulting from the oxidation of the metal in the process of being treated.

[0003] In order to eliminate these impurities harmful to the subsequent properties of the semi-finished product, the liquid aluminium is subjected to various treatments for the elimination of impurities. The most widely used of these treatments, which uses a combination of chemical reactions and flotation phenomena, consists in introducing into the bath, in the form of small bubbles, a so-called “treatment” gas, which may be inert or reactive. For example, a bubble of argon will carry with it to the surface of the bath a suspended solid inclusion and/or capture, by means of diffusion, the hydrogen dissolved in the liquid metal. In the same way, a bubble of chlorine will react with the sodium content and yield a sodium salt which will likewise be transported to the surface of the bath. Mixtures are also used, such as argon possibly containing several percent of a chlorine-type reactive gas.

[0004] Such gas-effect treatments may be carried out intermittently in a furnace or in a crucible (then referred to as a “batch” treatment). The treatments are most frequently carried out continuously between the furnace and the casting machine in a trough or in a treatment vat (or “ladle”) of the type shown schematically in FIG. 1.

[0005] The efficiency of the treatment is at a maximum when the exchange surface between the bath and the gas is itself at a maximum. This is obtained by designing the dispersion device so as to obtain very small bubbles, to spray these bubbles into the entire volume of liquid metal (i.e., in order to produce the least amount of dead space possible) and to create recirculating flows of the bath itself so that it comes into contact with the bubbles (always for the purpose of producing the least amount of dead space possible).

[0006] The treatment gas can be dispersed into the liquid metal in various ways. In general, static dispersion devices such as blowpipes are used or, more frequently, rotary dispersion devices which comprise one or more rotary injectors.

[0007] A rotary injector or “rotor” is typically composed of a hollow drive shaft through which the gas arrives, gas emitting orifices and blades. The blades serve to stir the bath, to disperse the gases within it and, sometimes, to split the bubbles into smaller size bubbles by means of a shearing effect. The orifices are generally situated in proximity to the rotor blades, e.g., between the latter or at their ends. International application WO 98/05915 (corresponding to the American patent U.S. Pat. No. 6,060,013) describes a rotary injector of this type.

[0008] European application EP 819 770 (equivalent to the American patent U.S. Pat. No. 5,904,894) describes a rotary injector in which the treatment gas is injected using a porous material that is inert with respect to the liquid metal.

[0009] The search for the greatest treatment efficiency by means of an intense agitation of the bath, resulting in a permanent surface agitation often called “surface waves”, which may cause the bath to spatter due to the surfacing of large bubbles and to a vortex phenomenon around the drive shaft, runs the risk of causing a “regassing” of the metal and degradation of the inclusion quality by forming surface oxides and/or by carrying inclusions or oxides from the free surface towards the interior of the liquid metal. Therefore, one seeks to limit the agitation of the free surface of the liquid metal as much as possible.

[0010] In addition, the recourse to heavy agitation of the liquid metal leads to the use of rotating speeds of the rotary injectors which generally range between 200 and 1,000 rpm depending on the type of injector. Such speeds bring about a large amount of wear on the moving parts of the gas dispersion device.

[0011] In addition, known rotary injectors do not enable the flow rate and size of the gas bubbles emitted to be controlled satisfactorily. Rotary injectors comprise emitting orifices subject to risks involving blockage of the orifices and changes in the size of the orifices and blades due to erosion, which alters the quality of the gas dispersion.

[0012] In the case where the rotary injectors comprise a porous material for dispersing the gas, the pores are often too large. Consequently, on the one hand, the bubbles are too large, lack efficiency, because the gas is insufficiently dispersed within the liquid metal, and cause harmful surface turbulence; on the other hand, it is necessary that the flow of the gas into the pores not be stopped, in order to prevent the liquid metal from penetrating therein, in particular during the non-operating periods between two castings. Conversely, when the pores are too small, the bubbles spread out and remain large and it is difficult to introduce a high flow rate into the liquid metal. Thus, several known methods using porous diffusers situated at the bottom of the vats or furnaces, having even very fine pore sizes (e.g., less than approximately 1 mm), make it possible to obtain, at best, bubbles of the order of 30 to 50 mm in diameter.

[0013] The applicant has pursued his efforts to increase the efficiency of liquid metal treatment devices, especially by controlling and reducing the diameter of the bubbles emitted by a rotary gas injector.

SUBJECTS OF THE INVENTION

[0014] The subject of the invention is a rotary injector for injecting gas, referred to as a “treatment gas,” into a liquid metal, comprising a drive shaft, stirring means, gas circulating means and means for emitting said gas, and being characterized in that the emitting means are entirely or partially made of at least one material capable of being wetted by said liquid metal, said material preferably being substantially inert to said liquid metal.

[0015] According to one advantageous variant of the invention, said material can be rendered wettable by means of a coating made of a material capable of being wetted by the liquid metal.

[0016] Another subject of the invention is a rotary dispersion device comprising at least one rotary injector according to the invention.

[0017] Another subject of the invention is a device for treating a liquid metal, such as a degassing ladle, comprising at least one rotary injector or at least one rotary dispersion device according to the invention.

[0018] Another object of the invention is the use of the injector according to the invention for treating a liquid metal in batches or continuously, especially in a furnace or a trough.

[0019] Another subject of the invention is a method for treating a liquid metal, characterized in that at least one rotary injector according to the invention is used.

[0020] Said metal may be aluminium or one of its alloys, or magnesium or one of its alloys.

DESCRIPTION OF THE FIGURES

[0021]FIG. 1 illustrates a typical device for treating liquid metal using a rotary injector.

[0022]FIG. 2 illustrates the wettability criterion as defined in the present invention.

[0023]FIG. 3 illustrates four embodiments of the rotary injector according to the invention, seen as a perspective view.

[0024]FIG. 4 illustrates two embodiments of the rotary injector according to the invention, seen through the axis of symmetry from the side of the part intended to be immersed in the liquid metal.

[0025]FIG. 5 illustrates an embodiment of the rotary injector according to the invention, seen in longitudinal section, in a cut-away plane passing through the axis of symmetry and corresponding to the cut-away planes B-B of FIG. 4.

[0026]FIG. 6 illustrates two embodiments of the rotary injector of the invention, seen in longitudinal section in a cut-away plane passing through the axis of symmetry.

[0027]FIG. 7 illustrates three embodiments of the rotary injector of the invention, seen in longitudinal section in a cut-away plane passing through the axis of symmetry.

DETAILED DESCRIPTION OF THE INVENTION

[0028] As illustrated in FIG. 1, a device for treating liquid metal 40 typically comprises an enclosure 41 equipped with inlet means 42 for the “raw” liquid metal (i.e., liquid metal to be treated) 421, outlet means 43 for the treated metal 431 and at least one rotary dispersion device 30. The inlet 42 and outlet 43 means are generally situated either at the ends of the device or on one and the same side.

[0029] A rotary dispersion device 30 typically comprises a rotary injector 1, means for rotating 31 said injector, a treatment gas source 32 and pipelines 33 between said source 32 and the injector 1. The rotary injector(s) 1 penetrate(s) into said enclosure 41 by way of an opening 44 which is generally equipped with sealing means 45. The treatment enclosure 41 is generally a vat with one or more compartments 46, 47.

[0030] According to the invention, the rotary injector 1 for injecting gas 2 into a liquid metal 3 comprises a drive shaft 4, stirring means 5, gas circulating means 6, 7, 11, means for emitting 8, 9 the gas 2, said injector being characterized in that the emitting means 8, 9 are entirely or partially made of at least one material capable of being wetted by the liquid metal 3.

[0031] Preferably, said wettable material is substantially inert to said liquid metal, i.e., it has a service life in the liquid metal that is sufficiently long to make it acceptable for industrial use. Typically, a material is considered as being substantially inert to the liquid metal when it can be immersed in the liquid metal for a period of approximately b 10 hours or more without any significant deterioration of the characteristics of the rotary injector and without any latent pollution of the treated metal. Ceramics generally meet this condition, especially ceramics based on oxides, carbides, nitrides, borides and mixtures thereof. Certain refractory metals likewise meet this condition, such as tungsten.

[0032] It is also possible to obtain a satisfactory service life, and possibly a reduction in the costs of manufacture and maintenance, by using a material coated with a product inert to the liquid metal that renders it capable of being wetted by the latter. In this case, the emitting means 8, 9 and/or stirring means 5 and/or said drive shaft 4 and/or said insert 90 comprise a coating made of a material that is wettable over all or part of its surface exposed to the liquid metal. Said porous material may likewise be rendered wettable by liquid metal by means of a coating made of a wettable material, i.e., it may comprise a coating made of a wettable material.

[0033] As defined by the present invention, a material is considered as being wettable when the wetting angle made by the liquid metal upon contact is less than 90° (see FIG. 2). When the material is capable of being wetted by the liquid metal (as in FIG. 2a), the wetting angle 21 between the tangent T to the bubble 20 at its point of contact with the emitting means 9 and the exterior surface S of the emitting means is less than 90°. In this case, the metal, which then wets the material well in the area of the emitting orifice 8, impedes the bubble 20 from spreading out and limits the diameter thereof. When the material is not capable of being wetted by the liquid metal (as in FIG. 2b), the wetting angle 21 is greater than 90°. In this case, the metal, which has difficulty wetting the emitting means, allows the bubble to spread out.

[0034] In the case of aluminium or magnesium or their liquid alloys, the wettable material of the diffuser may be selected from among certain refractory metals that are substantially inert with respect to said liquid metals, molybdenum (Mo), tungsten (W), vanadium (V), titanium (Ti), chromium (Cr), iron (Fe), steels, etc., or their alloys, or among ceramics such as titanium diboride (TiB₂), nitrides (in particular aluminium nitrides (such as AlN)), carbides (in particular aluminium carbides (such as Al₄C₃) and titanium carbides (such as TiC_(1−x))), . . . In this regard, it may be noted that graphite and aluminium oxide are normally not capable of being wetted by these liquid metals. ZrO₂ and SiC are also materials not capable of being wetted by aluminium and its alloys. In its tests, the applicant observed that boron nitride (BN) was not capable of being wetted by aluminium and its alloys. The wetting behavior of a material also depends on the roughness and oxidation state of its surface.

[0035] The emitting and stirring means, which constitute the so-called “active” part of the injector, are generally situated at the so-called “lower” end of the injector, i.e., the end of the injector intended to be immersed in the liquid metal. The injector is normally intended to be used in the vertical position, with said lower part placed towards the bottom. The active part normally comprises at least one lower surface 120, 121, 122, at least one upper surface 130, 131 and lateral surfaces 140, 141, 142.

[0036] As illustrated in FIG. 5, the circulating means 6, 7, 11 typically comprise a main channel 6 inside the shaft 4 of the injector and at least one secondary channel 7 for channeling the treatment gas to the emitting means 8, 9. The main channel 6 is typically within the axis of symmetry of said shaft.

[0037] In the preferred embodiment of the invention, said emitting means include at least one orifice 8 for emitting said gas 2. The diameter of the orifice 8 influences the diameter of the bubble to be obtained. In order to obtain small bubbles, the diameter of each orifice 8 is preferably as small as possible. In practice, the diameter preferably ranges between 0.5 and 5 mm, and more preferably between 1 and 3 mm, which makes it possible to control the size of the orifices at the time of their manufacture.

[0038] For diameters less than 0.5 mm, it is more advantageous to use wettable porous materials for which it is easy to control the sintering and formation of pores. In this case, said emitting means comprise a porous material capable of being wetted by said liquid metal 3, and preferably also substantially inert to said liquid metal 3, for which the diameter of the open pores appearing on the surface of said porous material is preferably less than 0.5 mm.

[0039] In order to better control the diameter of the bubbles, it is important that the gas pressure at the location of the emitting orifice 8 and/or pores appearing at the surface of the emitting means, at the interface between the metal and the surface of the emitting means 9, be substantially constant regardless of the gas flow rate, in particular during the formation and detachment of the bubble 20. To this end, the rotary injector 1 may also comprise an intermediate cavity 11, typically between the main channel 6 and the secondary channels 7, which serves as a buffer volume, and/or a means for introducing a local head loss just upstream from the emitting orifice, such as a porous material. The intermediate cavity 11 typically has a cylindrical shape and the secondary channels 7 radiate outward therefrom in the direction of the emitting means 8, 9.

[0040] The emitting orifices 8 are preferably situated in proximity to the injector blades 5, typically in-between them (FIGS. 3a and 3 b) or at their ends (FIGS. 3c and 3 d). Emitting orifices may be provided at the end of the injector; e.g., an orifice may be provided in the central part of said lower surface 120 of the injector. The number of emitting orifices 8 may differ from the number of blades 5. It is likewise possible to provide for superimposed emitting orifices. In practice, one emitting orifice is provided for each blade.

[0041] The emitting orifices 8 preferably open out onto said lateral surfaces 140, 141, 142, e.g., onto the external lateral surface 141 of one of the blades 5 or onto the lateral surface 140 between the blades. The position of the emitting orifices is preferably such that it makes it possible to obtain a maximum shearing of the bubbles as they are being formed. When the emitting orifices are situated between the blades, they are preferably situated halfway up the corresponding lateral surface 140; when the emitting orifices are situated on the blades, they may be situated in the upper half of the corresponding external lateral surface 141 (i.e., in the part of said surface closest to the shaft 4). The emitting orifices typically open out, in relation to the lateral surface, at an angle equal to approximately 90°; in certain cases, this angle may be different from 90°, in which case the axis of the secondary channels 7 may also form an angle in relation to the axis of the main channel which is different from 90°.

[0042] The stirring means 5 may also be entirely or partially made of at least one material capable of being wetted by said liquid metal 3, and preferably also substantially inert to said liquid metal 3, which material may be different from that used for the emitting means 8, 9. The stirring means typically comprise blades 5. These blades normally have a simple shape, such as a plate shape. The stirring means may also comprise a complementary dispersing means, such as a disk 12 situated above the blades, typically in contact therewith (as illustrated in FIGS. 3a, 3 c and 4 a).

[0043] Advantageously, the drive shaft 4 may be entirely or partially made of at least one material capable of being wetted by said liquid metal, and preferably substantially inert to said liquid metal, which material may be different from that used for the emitting means 8, 9. In practice, it suffices for at least the surface of the part of said shaft intended to be immersed in said liquid metal to be made of said wettable material.

[0044] In order to facilitate manufacture, maintenance and repairs, the rotary injector 1 according to the invention may be composed of several separate parts 4, 5, 12, 13, 14, 90, as illustrated in FIGS. 6 and 7. The parts may be made of different materials. In particular, the rotary injector may advantageously include an insert 90 comprising said emitting means 8, 9 and made of said wettable material, which makes it possible to easily change it depending on the metal being treated or in case of accidental breakage. The part of the injector intended to be immersed in the liquid metal may be made of a single piece.

[0045] As a non-limiting example illustrated in FIG. 6a, the injector comprises the following parts: a drive shaft 4, a disk 12, blades 5, a central core 13 and an assembly piece 14. The central core comprises an intermediate cavity 11, circulating channels 7 and emitting orifices 8. In the embodiment illustrated in FIG. 6b, the injector comprises the following parts: a drive shaft 4, blades 5, and an assembly piece 14. The blades comprise circulating channels 7, emitting orifices 8 and an intermediate cavity 11, said cavity generally being common to all of the blades and enclosed inside a central core 13 (not shown). In these two embodiments, the assembly piece 14 comprises at least one central channel 60 and connecting means 15 a, 16 a, 17 a, typically a thread, which cooperate with complementary connecting means 15 b, 16 b, 17 b of the other parts 4, 12, 13. According to possible variants of these embodiments, illustrated in FIG. 7, the central core 13 and/or blades 5 are equipped with removable inserts 90.

[0046] According to the invention, it suffices if only the emitting means 9 are made of a wettable material. The applicant's tests have shown that it was particularly advantageous for all of the parts of the injector immersed in the liquid metal during the treatment to be made of a wettable material. The same material may be used for all of these parts. In fact, it has been observed that, in this case, the bubbles emitted by the orifices 8 which are pulled into the blades and along the shaft of the rotor by a hydrodynamic effect do not remain trapped and do not tend to merge together to form larger size bubbles, as is the case with non-wettable materials. When the injector is composed of several pieces, the pieces of the injector that are immersed in the liquid metal during the treatment are preferably all made of a wettable material. The same material may be used for all of these pieces.

[0047] The injector may be equipped with a ring 10 to enable coupling with rotating means 31.

[0048] The axis of rotation of the rotary injector 1 is situated in the axis of symmetry of the drive shaft 4.

[0049] The rotary injector 1 of the invention may be used for treating a liquid metal circulating inside an enclosure, as illustrated in FIG. 1, which is typically a treatment ladle, or in a liquid metal circulation trough (not shown). It may also be used for batch treatments, e.g., in a furnace. In other words, a treatment ladle, a furnace or a trough may be equipped with a rotary injector according to the invention with a view to treating a liquid metal continuously or in batches (batch treatment).

EXAMPLES

[0050] Tests were carried out in a small-sized experimental vat. The size of the bubbles formed was observed and determined by means of an X-ray camera. The method consists in using X-rays to irradiate the liquid metal bath 3, into which the bubbles 20 are emitted, in viewing said bubbles after recovering the image by means of a camera, and in measuring them after calibration of the acquisition chain.

[0051] The tests were carried out with rotary injectors comparable to those illustrated in FIG. 3. In one case representative of the prior art, the blades and emitting means were made of graphite; in another case representative of the invention, they were made of titanium. In both cases, the orifices measured 1 mm in diameter.

[0052] In these tests, the applicant observed that, on the one hand, with the prior art injectors the bubbles had an average diameter of the order of 15 mm, a portion of the treatment gas was able to rise along the rotor and injector shaft and 20% of the injected gas was not dispersed within the liquid metal. The non-dispersed portion of the gas is practically useless because it does not contribute to the treatment of the liquid metal.

[0053] On the other hand, the applicant observed that, with injectors according to the invention, the bubbles had an average diameter of the order of 6 mm and less than 0.5% of the injected gas (lower limit of detection) was not dispersed within the liquid metal.

[0054] The applicant also noted that, contrary to the prior art, the bubbles emitted by the orifices situated at the ends of the blades do not have a tendency to form pockets of gas between the blades. The bubbles therefore maintain their small size, which results in a higher treatment efficiency than in the prior art.

[0055] The applicant also noted that the injector according to the invention prevents the formation of gas pockets underneath the injector, which might cause instabilities.

[0056] Thus, the degassing performance obtained using injectors according to the invention is clearly improved over that observed when using prior art injectors.

ADVANTAGES OF THE INVENTION

[0057] The rotary injector according to the invention has the advantage of enabling a significant reduction in the rotating speed required to obtain small-sized bubbles by means of a shearing effect. Using an injector according to the invention and for an output equivalent to that of the prior art, the rotating speed can range between 100 and 350 rpm, which also makes it possible to limit the surface agitation of the liquid metal and to reduce wear of the parts.

[0058] The rotary injector according to the invention also has the advantage of offering treatment performances that are less sensitive to the possible wear of the injector blades. Actually, according to the invention, the size of the gas bubbles is to a very large extent determined by the emitting orifices, and only to a small extent by the rotary motion of the blades, which then have the primary functions of dispersing the bubbles into the largest possible bath volume and of stirring it, especially with a view to making the treatment uniform. Consequently, the wear of the blades over time does not bring about any latent deterioration in the performance of the injector according to the invention.

[0059] The orifice of the emitting means of the injector according to the invention may be sufficiently small so as to prevent the penetration of liquid metal. 

1. Rotary injector (1) for injecting gas (2) into a liquid metal (3), comprising a drive shaft (4), stirring means (5), means (6, 7, 11) for circulating said gas (2) and means (8, 9) for emitting the gas (2), and characterized in that the emitting means (8, 9) are entirely or partially made of at least one material capable of being wetted by said liquid metal (3).
 2. Rotary injector (1) according to claim 1, characterized in that said wettable material is also substantially inert to said liquid metal (3).
 3. Rotary injector (1) according to claim 1 or 2, characterized in that said material is rendered wettable by means of a coating made of a material capable of being wetted by the liquid metal.
 4. Rotary injector (1) according to any one of claims 1 to 3, characterized in that said emitting means include at least one orifice (8) for emitting said gas (2).
 5. Rotary injector (1) according to claim 4, characterized in that the diameter of said or each orifice (8) ranges preferably between 0.5 and 5 mm.
 6. Rotary injector (1) according to any one of claims 1 to 5, characterized in that said emitting means comprise a porous material capable of being wetted by said liquid metal (3) or a porous material rendered wettable by means of a coating made of a material capable of being wetted by the liquid metal.
 7. Rotary injector (1) according to claim 6, characterized in that the diameter of the open pores appearing at the surface of said porous material is less than 0.5 mm.
 8. Rotary injector (1) according to any one of claims 1 to 7, characterized in that it includes an insert (90) comprising said emitting means (8, 9) and made of said wettable material or a material rendered wettable by means of a coating made of a material capable of being wetted by the liquid metal.
 9. Rotary injector according to any one of claims 1 to 8, characterized in that said stirring means (5) are also entirely or partially made of at least one material capable of being wetted by said liquid metal or a material rendered wettable by means of a coating made of a material capable of being wetted by the liquid metal.
 10. Rotary injector (1) according to any one of claims 1 to 9, characterized in that said drive shaft (4) is also entirely or partially made of at least one material capable of being wetted by said liquid metal or a material rendered wettable by means of a coating made of material capable of being wetted by the liquid metal.
 11. Rotary injector (1) according to any one of claims 1 to 10, characterized in that the wettable material or materials are selected from amongst the group comprising molybdenum, tungsten, vanadium, titanium, chromium, iron, steels or their alloys, and titanium diboride, aluminium nitrides, aluminium carbides, and titanium carbides.
 12. Rotary dispersion device (30), characterized in that it comprises at least one rotary injector according to one of claims 1 to
 11. 13. Device for treating a liquid metal (40), characterized in that it comprises at least one rotary injector according to any one of claims 1 to 11 or at least one rotary dispersion device according to claim
 12. 14. Ladle for degassing a liquid metal comprising at least one rotary injector according to any one of claims 1 to 11 or at least one rotary dispersion device according to claim
 12. 15. Furnace comprising at least one rotary injector according to any one of claims 1 to 11 or at least one rotary dispersion device according to claim
 12. 16. Trough equipped with at least one rotary injector according to any one of claims 1 to 11 or at least one rotary dispersion device according to claim
 12. 17. Use of a rotary injector according to any one of claims 1 to 11, a rotary dispersion device according to claim 12, a treatment device according to claim 13, a degassing ladle according to claim 14, a furnace according to claim 15 or a trough according to claim 16 for treating a metal in the liquid state.
 18. Use according to claim 17, characterized in that said metal is selected from amongst aluminium, aluminium alloys, magnesium and magnesium alloys.
 19. Method for treating a metal in the liquid state, characterized in that use is made of at least one rotary injector according to any one of claims 1 to 11, at least one rotary dispersion device according to claim 12, at least one degassing device according to claim 13, at least one degassing ladle according to claim 14, at least one furnace according to claim 15 or at least one trough according to claim
 16. 20. Treatment method according to claim 19, characterized in that said metal is selected from amongst aluminium, aluminium alloys, magnesium and magnesium alloys. 